The present invention relates to pulse duration amplifiers and, more particularly, to improvements therein.
Pulse duration modulation (PDM) is frequently employed in power amplifiers to achieve efficiency of operation. In such amplifiers, an input signal, such as an audio signal, is used to pulse modulate a carrier signal, and the resulting PDM signal is amplified. The amplified PDM signal is then filtered to recover a modulated signal corresponding to an amplified version of the input audio signal.
A polyphase PDM amplifier is disclosed in U.S. Pat. Nos. 4,164,714, 4,468,626 and 5,216,376 to H. Swanson and assigned to the assignee herein. These patents disclose a system wherein an amplified signal is formed by combining the outputs of plural parallel-connected PDM amplifiers. All of the amplifiers respond to a common input signal, but have differently phased carrier signals. Polyphase PDM amplifiers of this type enjoy various advantages over earlier, single phase PDM amplifiers. For example, amplifier elements employed in polyphase PDM amplifiers operate at relatively low switching frequencies and power levels, permitting semi-conductor devices to be used as the active amplifier elements instead of vacuum tube devices.
The switching frequencies at which the PDM amplifiers operate serve to drive a switching device such as a semi-conductor to generate a train of squarewave pulses of fixed magnitude but variable pulse width in dependence on the on-off time durations of the semi-conductor switching device.
In the PDM amplifier systems described in the noted patents, each of the PDM amplifiers employs a semi-conductor switching device. Each switching device, in turn, supplies output PDM pulses to an output circuit which includes a low-pass filter which filters the amplified PDM output signal pulses to recover an amplified input signal. This filter includes an inductor through which current continuously flows. The maximum cutoff frequency of the filter is proportional to the PDM pulse frequency. For example, a prior art PDM amplifier system constructed in accordance with the noted patents may have a PDM frequency on the order of 70 KHz. In such case, the maximum cutoff frequency is about 20 KHz and the maximum usable frequency is about 10 KHz. It is to be noted that such a filter is sensitive to load impedance changes at frequencies near its cutoff frequency and therefore if the cutoff frequency can be increased the filter will be less sensitive to the changes. Also, instead of increasing the cutoff frequency of the filter the PDM frequency can be decreased which will permit use of lower cost devices, such as IGBTs.